The present invention relates to a thixocast Fe-based alloy material, and a process for heating the same.
In carrying out a thixocasting process, a procedure is employed which comprises heating an Fe-based alloy material into a semi-molten state in which a solid phase (a substantially solid phase and this term will also be applied hereinafter) and a liquid phase coexist, pouring the semi-molten Fe-based alloy material under a pressure into a cavity in a casting mold, and solidifying the semi-molten Fe-based alloy material under a pressure.
There is such a known Fe-based alloy material having a eutectic crystal amount Ec set in a range of 50% by weightxe2x89xa6Ecxe2x89xa670% by weight (see Japanese Patent Application Laid-open No.5-43978).
However, if the eutectic crystal amount Ec is set to be equal to or larger than 50% by weight, the amount of graphite precipitated is increased in such an Fe-based alloy material, and hence, the mechanical properties of a cast product are substantially equivalent to those of a cast product made by casting. Therefore, with the conventional material, it is impossible to achieve an intrinsic purpose of enhancing the mechanical properties of the cast product made by the thixocasting process.
In a quenched area such as a thinner portion in the cast structure of the cast product, a portion which has been a spherical solid phase is transformed into a mixed structure of austenite and martensite. On the other hand, in a slowly cooled area such as a thicker portion, a portion which has been a spherical solid phase is transformed into a pearlite structure. Portions which have been liquid phases in both the areas are transformed into a ledeburite structure (a chilled structure).
If such a cast product is subjected to a thermal treatment, the following problem also arises: Graphite is finely precipitated in the quenched area, while it is precipitated in a coalesced manner in the slowly cooled area. As a result, the mechanical properties of both the areas are different from each other. For this reason, it is impossible to produce a cast product having mechanical properties uniform over the whole thereof.
Further, in the thixocasting process, the temperature of the semi-molten Fe-based alloy material, namely, the casting temperature is low as compared with the temperature of a molten metal. Therefore, when a cast product having a smaller thickness or having a complicated shape is produced by casting, the semi-molten Fe-based alloy material is cooled rapidly by the casting mold, and as a result, a portion which has been a liquid phase has a chilled structure having a low toughness. The chilled structure is liable to become a starting point for cracking on the solidification and shrinkage of the material, which is undesirable. Therefore, a measure to form an inner wall of a casting mold from a carbon material such as graphite is employed to moderate the quenching of the material. However, the following problem is encountered by utilizing the thixocasting process: The carbon material is worn violently and for this reason, the replacement of the casting mold must be performed frequently, which is uneconomic, and moreover, which results in a reduced productivity.
On the other hand, if the stability and productivity of components and metallographic structure and the like of the Fe-based alloy material are taken into consideration, it is optimal to produce such material by a continuous casting process. In the continuous casting process, however, the cooling rate of the Fe-based alloy material is high, and for this reason, a chilled structure may be produced in the material in some cases. When such an Fe-based alloy material is heated, the following problem arises: The temperature gradient of the inside of the material is increased depending on heating conditions, whereby cracks are produced in the material, and the material cannot be heated to a target temperature during induction-heating.
Accordingly, it is an object of the present invention to provide a thixocast Fe-based alloy material of the above-described type, from which a cast product having mechanical properties which are more excellent than those of a cast product made by casting, and which are uniform over the whole of the cast product, can be produced.
To achieve the above object, according to the present invention, there is provided a thixocast Fe-based alloy material comprising
1.8% by weightxe2x89xa6Cxe2x89xa62.5% by weight,
1.0% by weightxe2x89xa6Sixe2x89xa63.0% by weight,
0.1% by weightxe2x89xa6Mnxe2x89xa61.5% by weight,
0.5% by weight greater than Nixe2x89xa63.0% by weight, and
as the balance, iron (Fe) including inevitable impurities, and wherein a eutectic crystal amount Ec is in a range of 10% by weight less than Ec less than 50% by weight.
A semi-molten Fe-based alloy material having liquid and solid phases coexisting therein is prepared by subjecting the Fe-based alloy material having the above composition to a heating treatment. In this semi-molten Fe-based alloy material, the liquid phase produced by a eutectic melting has a large latent heat. As a result, in the course of solidification of the semi-molten Fe-based alloy material, the liquid phase is supplied in a sufficient amount around the solid phase in response to the solidification and shrinkage of the solid phase, and is then solidified. Therefore, the generation of voids of a micron order in the cast product is prevented. In addition, the amount of graphite precipitated can be reduced by setting the eutectic crystal amount Ec in the above-described range. Thus, it is possible to enhance the mechanical properties, i.e., the tensile strength, the Young""s modulus, the fatigue strength and the like of the cast product. In the Fe-based alloy material with the eutectic crystal amount Ec in the above-described range, it is possible to lower the casting temperature of the Fe-based alloy material, thereby providing the prolongation of the life of a casting mold.
However, if the eutectic crystal amount Ec is equal to or smaller than 10% by weight, the casting temperature of the Fe-based alloy material approximates to a liquidus temperature due to the small eutectic crystal amount Ec. Therefore, a heat load of a material transporting equipment to a pressure casting apparatus is high, thereby making it impossible to carry out the thixocasting. On the other hand, a disadvantage raised when Ecxe2x89xa750% by weight is as described above.
In the above-described composition, manganese (Mn) is a cementite and austenite producing element, and nickel (Ni) is an austenite producing element. Therefore, Mn and Ni inhibit the slowly cooled area from being transformed into a pearlite structure. Thus, the cast structure of the entire cast product is such that a portion which has been a solid phase is transformed into a mixed structure of austenite and martensite, and a portion which has been a liquid phase is transformed into a ledeburite structure.
By subjecting such a cast product into a predetermined thermal treatment, a cast product having a uniformly thermally treated structure with fine graphite dispersed in a mixed structure of ferrite and pearlite is produced. This cast product has mechanical properties uniform over the whole thereof.
In the above-described composition, carbon (C) and silicon (Si) participate in the eutectic crystal amount, and the C content and the Si content are set in the above-described ranges to control the eutectic crystal amount in the above-described range. However, if the C content is smaller than 1.8% by weight, the casting temperature must be high, even if the Si content is increased to increase the eutectic crystal amount. Therefore, the advantage of the thixocasting is degraded. On the other hand, if C greater than 2.5% by weight, the amount of graphite is increased. For this reason, the mechanical properties of the cast product is degraded, and the eutectic crystal amount is increased and hence, the handlability of the semi-molten Fe-based alloy material is deteriorated. If the Si content is smaller than 1.0% by weight, the casting temperature is raised as the case where the C content is smaller than 1.8% by weight. On the other hand, if Si greater than 3.0% by weight, silico-ferrite is produced and for this reason, the mechanical properties of the cast product cannot be enhanced.
Manganese (Mn) functions as a deoxidizing agent and is required for producing cementite. However, if the Mn content is smaller than 0.1% by weight, the deoxidizing effect is smaller and for this reason, defects due to inclusion of an oxide caused by the oxidation of the molten metal and due to bubbles are liable to be produced. On the other hand, if Mn greater than 1.5% by weight, the amount of cementite [(FeMn)3C] crystallized is increased. For this reason, it is difficult to finely divide the large amount of cementite by a thermal treatment, resulting in a reduced toughness and a reduced cutting property of a cast product.
Nickel (Ni) is an austenite producing element, as described above, and has an effect which allows austenite to exist in a very small amount at normal temperature to enclose impurities in the austenite, thereby enhancing the toughness. To provide such effect, it is necessary to set the Ni content at about 1% by weight. However, if the Ni content is smaller than 0.5% by weight, the addition of nickel is meaningless. On the other hand, if Ni greater than 3.0% by weight, amatrix is transformed into a martensite structure with an increased hardness in the course of cooling following a cementite-eliminating thermal treatment.
It is another object of the present invention to provide a thixocast Fe-based alloy material of the above-described type, wherein the generation of cracks in a thin cast product and the like can be avoided.
To achieve the above object, according to the present invention, there is provided a thixocast Fe-based alloy material comprising
1.8% by weightxe2x89xa6Cxe2x89xa62.5% by weight
1.0% by weightxe2x89xa6Sixe2x89xa63.0% by weight
0.8% by weightxe2x89xa6Mnxe2x89xa61.5% by weight, and
as the balance, iron (Fe) including inevitable impurities, and wherein a eutectic crystal amount Ec being in a range of 10% by weight less than Ec less than 50% by weight.
When a thixocasting is carried out using the Fe-based alloy material having the above composition and using a conventional casting mold, a portion which has been a solid phase is transformed into a mixed structure of austenite and martensite in the entire thin cast product due to the presence of Mn which is an austenite producing element, and a portion which has been a liquid phase is transformed into a ledeburite structure. In this way, the toughness of the entire structure is enhanced by the austenite remaining in the portion which has been the solid phase. Therefore, in the thin cast product and the like, the generation of cracks due to the solidification and shrinkage is avoided. In addition, it has been made clear that if the above Fe-based alloy material is used, the pearlite transformation of a thick portion cooled at a low speed in the cast product can be inhibited to ensure that austenite remains in the portion which has been the solid phase.
In the alloy composition of this material, manganese (Mn) is an austenite producing element and has an effect of permitting austenite to remain in the portion which has been the solid phase, as described above. If the Mn content is smaller than 0.8% by weight, the amount of austenite remaining in the portion which has been the solid phase is insufficient, and the amount of austenite crystallized in ledeburite presenting a chilled structure is also insufficient. On the other hand, if Mn  greater than 1.5% by weight, the amount of cementite [(FeMn)3C] precipitated in ledeburite is increased, resulting in reduced toughness and cutting property of a product. Mn also has a function as a deoxidizing agent.
The reason why the eutectic crystal amount Ec, the C content and the Si content are limited in the Fe-based alloy material is the same as described above.
In addition, according to the present invention, there is provided a thixocast Fe-based alloy material, comprising carbon (C) of a content in a range of 1.8% by weightxe2x89xa6Cxe2x89xa62.5% by weight, silicon (Si) of a content in a range of 1.0% by weightxe2x89xa6Sixe2x89xa63.0% by weight, manganese (Mn) of a content in a range of 0.6% by weightxe2x89xa6Mnxe2x89xa61.5% by weight, at least one of nickel (Ni) of a content in a range of 0.2% by weightxe2x89xa6Nixe2x89xa63.0% by weight and titanium (Ti) of a content in a range of 0.05% by weightxe2x89xa6Tixe2x89xa60.6% by weight, the total sum of the Mn content, the Ni content and the Ti content being equal to or larger than 0.8% by weight (Mn+Ni+Tixe2x89xa70.8% by weight), and the balance of iron (Fe) including inevitable impurities, a eutectic crystal amount Ec being in a range of 10% by weight less than Ec less than 50% by weight.
If the Fe-based alloy material having the above composition is used, the generation of cracks due to the solidification and shrinkage can be further reliably avoided in a thin cast product.
The reason why the eutectic crystal amount, the C content and the Si content are limited in the Fe-based alloy material is the same as described above.
Nickel (Ni), which is an austenite producing element, acts to further promote the remaining of austenite and to enclose impurities in the remaining austenite for harmlessness. Namely, nickel (Ni) has an effect of dispersing the impurities reducing the toughness into the austenite rich in toughness, thereby preventing the impurities from influencing the mechanical properties. In addition, nickel (Ni) also has an effect of preventing the pearlite transformation of a portion cooled slowly such as a thick portion. However, If the Ni content is smaller than 0.2% by weight, the addition of nickel is meaningless. On the other hand, if the Ni content is larger than 3.0% by weight, when the cast product is subjected to a thermal treatment in order to ensure that cementite disappears, thereby forming spherical fine graphite grains, the precipitated graphite grains are agglomerated at points at points to bring about a reduction in toughness. In addition, the matrix is transformed into martensite by the cooling carried out after the thermal treatment, resulting in an increased hardness. Further, the addition of an excessive amount of nickel brings about an increase in material cost.
Titanium (Ti) has an effect.of finely dividing the crystal grains in the solid phase to further enhance the toughness of the cast product. However, if the Ti content is smaller than 0.05% by weight, the addition of titanium is meaningless. On the other hand, if Ti greater than 0.6% by weight, TiC is precipitated and for this reason, the cutting property is reduced and the flowability of the molten metal is reduced, resulting in the generation of casting defects.
The lower limit value of the Mn content may be decreased down to 0.6% by weight, lower than that of the Fe-based alloy material, because of the containment of titanium (Ti) and/or nickel (Ni). The reason why the upper limit value of the Mn content is limited is the same as described above.
Even in a casting process by casting, it is possible to allow austenite to remain, but for this purpose, the cooling rate must be managed extremely severely. According to the present invention, the remaining of austenite in a portion which has been a solid phase has been realized in the thixocasting process by specifying the total amount of the Mn content and the Ni and Ti contents (or the Ni or Ti content). A lower limit value of the total amount of the Mn content and the Ni and Ti contents (or the Ni or Ti content), 0.8% by weight, is a condition for providing the above-described effect without being influenced by the cooling rate.
It is desirable that the solid phase rate R in the semi-molten Fe-based alloy material in the thixocasting process is larger than 50%. This makes it possible to shift the casting temperature to a lower level to prolong the life of a pressure casting apparatus. If the solid phase rate R is equal to or smaller than 50%, the amount of the liquid phase is increased. For this reason, when a short columnar semi-molten Fe-based alloy material is transported in a standing state, the self-standing property thereof is degraded, and the handlability thereof is also degraded.
Further, it is an object of the present invention to provide a heating process, by which a thixocast Fe-based alloy material having a chilled structure can be heated into a semi-molten state without generation of cracks in the material.
To achieve the above object, according to the present invention, there is provided a process for heating a thixocast Fe-based alloy material having a chilled structure into a semi-molten state in which solid and liquid phases coexist, wherein the average rate HR of heating to a point Al in an Fexe2x80x94C based equilibrium diagram is set in a range of 0.5xc2x0 C./secxe2x89xa6HRxe2x89xa66.0xc2x0 C./sec, and the maximum temperature gradient TG of the inside of the Fe-based alloy material per unit distance is set at TGxe2x89xa67xc2x0 C./mm.
The average rate HR of heating to a point A1 and the maximum temperature gradient TG are specified as described above, the cracking due to the heating of the Fe-based alloy material having the chilled structure can be prevented, and the oxidation of the material and the coalescence of crystal grains cannot occur. After the temperature exceeds the point A1, the heating rate is increased to effect the decomposition of dendrite and the spheroidization of the solid phase. At this time, a xcex3-phase appears in the Fe-based alloy material, resulting in an enhanced toughness of the material. Therefore, even if the heating rate is increased, cracks cannot be produced in the Fe-based alloy material.
Both of 6.0xc2x0 C./sec which is an upper limit value for the average heating rate HR and 7xc2x0 C./mm which is an upper limit value for the maximum temperature gradient TG are limit values for preventing the generation of cracks due to the heating. If the average heating temperature HR is lower than 0.5xc2x0 C./sec, problems of a reduction in producibility of a cast product, the coalescence of the solid phases and the oxidation of the material surface arise.
The Fe-based alloy material which is the subject of the present invention is not limited to a material produced by a continuous casting process, and may be a material produced by casting and having a chilled structure.
To determine whether the Fe-based alloy material has a chilled structure, it is a common practice to observe the material by a metal microscope, but it is convenient to use an ultrasonic velocity measuring process which is one of non-destructive inspecting processes for a metal. The sonic velocity Sv measured by the ultrasonic velocity measuring process is in a range of 5,800 m/secxe2x89xa6Svxe2x89xa66,000 m/sec in a case of a steel. On the other hand, the reviews by the present inventors have made it clear that in a thixocast graphite-crystallized Fe-based alloy material, a flake-formed graphite phase is reflected as a defect to a measured value of sonic velocity and hence, the sonic velocity Sv assumes a low value in a range of 5,100 m/secxe2x89xa6Svxe2x89xa65,450 m/sec, but in an Fe-based alloy material having a chilled structure, the sonic velocity assumes a value near that of a steel due to non-precipitation of graphite. Therefore, it can be determined from such a difference between the sonic velocities that if the sonic velocity Sv measured for the Fe-based alloy material by the ultrasonic velocity measuring process is a valuexe2x89xa65,600 m/sec, this material is an Fe-based alloy material having a chilled structure.